1. Field of the Art
The present invention relates in general to a method and an apparatus for controlling a drive force of an automotive vehicle equipped with a continuously variable transmission, and more particularly to techniques for improving drivability of such a vehicle.
2. Related Art Statement
In an automotive vehicle equipped with a continuously variable transmission for transmitting the output of an engine to drive wheels, the speed ratio of the transmission, and the output of the engine if necessary, may be adjusted so that the engine may be operated to meet a minimum fuel-consumption curve or maximum fuel-economy curve which represents a relation between the output torque and speed of the engine. Hence, the vehicle provides a relatively high level of fuel economy. An example of an apparatus to control such engine and transmission is disclosed in Japanese Patent Application No. 57-40747 laid open in 1983 under Publication No. 58-160661.
3. Problem Solved by the Invention
While such a conventional control apparatus enables the vehicle to provide a high fuel economy, the apparatus is not capable of providing a satisfactorily high degree of drivability while the engine is operating in a transient state (e.g., while the vehicle is accelerated), because the conventional control arrangement is adapted to control the speed ratio of the continuously variable transmission and other variables, based on the characteristics of the engine which appear while the engine is operating in a steady state. In other words, the conventional arrangement is not capable of controlling a rate of variation in the speed ratio of the transmission so as to assure a high level of drivability while the engine is in a transient operating state, that is, the timing to change the speed ratio of the transmission tends to be too early or too late to meet the requirement for attaining the satisfactory drivability. For example, if the speed ratio "e" is changed at a relatively high rate, a time (t0-t1) between the start (t0) of operation of a throttle valve and the moment (t1) at which a desired drive force is obtained, may be reduced accordingly, as indicated in FIG. 19. However, the drive torque is not sufficient during this period of time. Therefore, the operator of the vehicle feels, at an initial stage of acceleration of the vehicle, a poor response of the engine to the operation of an accelerator pedal to accelerate the vehicle, and tends to depreciate the drivability of the vehicle. In FIG. 19, a broken line near a solid-line curve indicating the drive torque T represents an ideal curve for obtaining the desired drivability, and a broken line near a solid-line curve indicating the input shaft torque Ti of the transmission represents a required output torque Te of the engine which corresponds to the operating amount of the accelerator pedal of the vehicle.
On the other hand, if the speed ratio "e" of the continuously variable transmission is changed at a relatively low rate as indicated in FIG. 20, the drive torque immediately after the start (t0) of the throttle valve is relatively high. However, a time (t0-t2) after the start of the throttle valve movement is extended until the desired drive torque is obtained. Therefore, the operator feels a limited acceleration capability, and is apt to depreciate the drivability of the vehicle.
The unfavourable phenomena indicated above referring to FIGS. 19 and 20 may be explained as follows:
The drive torque To of the vehicle is expressed by the following equation: EQU To(t)=Ti(t)/e(t)=[Te(t)-I.multidot.Ni]/e(t) (1)
where,
I: Moment of inertia of the engine and the input shaft of the transmission PA1 Ni: Acceleration of the input shaft of the transmission
In the case of FIG. 19, the acceleration Ni of the input shaft of the continuously variable transmission is increased as the speed ratio e(t) is rapidly varied. Consequently, a variation in the drive torque To is reduced. In the case of FIG. 20, the situation is reversed.
According to the conventional control apparatus, the drive torque of the vehicle will be reduced in proportion to a decrease of the engine performance or output or a decrease of the transmission efficiency of the transmission due to changes of their characteristics during a long service. Hence, the drivability will be degraded as the service period of the vehicle is prolonged.